Field
The present disclosure relates generally to wireless communication systems and, more specifically, to mixer radio frequency (RF) input port reflection coefficient (S11) control using sum component termination in a downconverter mixer of a receiver.
Background
A wireless device (e.g., a cellular phone or a smartphone) in a wireless communication system may include a radio frequency (RF) transceiver to transmit and receive data for two-way communication. A mobile RF transceiver may include a transmit section for data transmission and a receive section for data reception. For data transmission, the transmit section may modulate an RF carrier signal with data to obtain a modulated RF signal, amplify the modulated RF signal to obtain an amplified RF signal having the proper output power level, and transmit the amplified RF signal via an antenna to a base station. For data reception, the receive section may obtain a received RF signal via the antenna and may amplify and process the received RF signal to recover data sent by the base station.
The basic operation of a receiver in a wireless device including a mobile RF transceiver is as follows. In the receiver, a signal from the antenna is amplified in an RF stage. The output of the RF stage is one input of an RF mixer. A local oscillator (LO) is the other input of the RF mixer. The output of the mixer is an intermediate frequency (IF) signal. A second mixer stage may convert the IF signal to a baseband signal. The signal is then demodulated. The modulation technique is independent from the receiver technology.
An RF mixer is a three port electronic circuit that converts a signal from one frequency to another. The three signal connections (or ports) of the RF mixer include an RF input, a local oscillator (LO) input, and an intermediate frequency (IF) output. In operation, the RF mixer takes a radio frequency (RF) input signal and mixes the RF input signal with the local oscillator input. Mixing of the RF input signal and the local oscillator input signals produces an IF output signal that includes a sum component and a difference component. The sum component has a frequency that is a sum of a frequency of the RF input signal and a frequency of the local oscillator input signal. The difference component has a frequency that is a difference between the RF input signal frequency and the local oscillator input signal frequency.
An RF mixer may be referred to as a downconverter if the RF mixer is part of a receiver or as an upconverter if the RF mixer is part of a transmitter. When the desired frequency is less than the second input frequency, the process is called downconversion. The RF signal is then the input while the IF signal is the output. When the desired output frequency is greater than the second input frequency, the process is called upconversion. In this case, the IF signal is the input while the RF signal is the output. In a receiver, when the LO frequency is less than the RF frequency, it is called low-side injection and the mixer is a low-side downconverter. When the LO frequency is above the RF frequency, it is called high-side injection, and the mixer is a high-side downconverter.
In RF applications, usually a desired noise match does not coincide with a power match. This means that a receiver design often has to compromise between a noise figure (NF) and an input return loss (S11). While techniques exist for compromising between a noise figure and an input return loss when using a low noise amplifier (LNA), these same techniques cannot be used in mixer-first designs, which do not include an LNA.